A study of mammalian fertilization was undertaken to investigate (1) various aspects of fertilization in vivo and in vitro of different laboratory animals, (2) the morphology, physiology and biochemistry of mammalian eggs before, during and after fertilization, (3) the possibility and conditions under which fertilization can be achieved between different mammalian species, 4) the mechanism of sperm penetrating and 5) the possibility of inhibition of fertilization in vitro and in vivo. BIBLIOGRAPHIC REFERENCES: Fertilization of rat eggs in vitro at various times before and after ovulation with special reference to fertilization of ovarian oocytes matured in culture, by K. Niwa and M.C. Chang, J. Reprod. Fert. 43: 435-451, 1975. Requirement of capacitation for sperm penetration of zona-free rat eggs, by K. Niwa and M.C. Chang, J. Reprod. Fert. 44: 305-308, 1975.